[Xen-changelog] [xen-4.2-testing] xen/mm/shadow: check toplevel pagetables are present before unhooking them.

[Xen patchbot-4.2-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1352893365 0
# Node ID 159080b58dda9d19a5d3be42359e667bdb3e61ca
# Parent  4cffe28427e0c7dbeaa7c109ed393dde0fe026ba
xen/mm/shadow: check toplevel pagetables are present before unhooking them.

If the guest has not fully populated its top-level PAE entries when it calls
HVMOP_pagetable_dying, the shadow code could try to unhook entries from
MFN 0.  Add a check to avoid that case.

This issue was introduced by c/s 21239:b9d2db109cf5.

This is a security problem, XSA-23 / CVE-2012-4538.

Signed-off-by: Tim Deegan <tim@xxxxxxx>
Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

xen-unstable changeset: 26150:c7a01b6450e4
Backport-requested-by: security@xxxxxxx
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---


diff -r 4cffe28427e0 -r 159080b58dda xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:36:57 2012 +0000
+++ b/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:42:45 2012 +0000
@@ -4734,8 +4734,12 @@ static void sh_pagetable_dying(struct vc
         unsigned long gfn;
         mfn_t smfn, gmfn;
 
-        if ( fast_path )
-            smfn = _mfn(pagetable_get_pfn(v->arch.shadow_table[i]));
+        if ( fast_path ) {
+            if ( pagetable_is_null(v->arch.shadow_table[i]) )
+                smfn = _mfn(INVALID_MFN);
+            else
+                smfn = _mfn(pagetable_get_pfn(v->arch.shadow_table[i]));
+        }
         else
         {
             /* retrieving the l2s */

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